Thank you Nishant. I tried to wait for ACK from the device by CyI3PI2cWaitForAck but I could not solve it. I researched on detail and noticed that it depended on the physical condition. In this case, USB3.0 cable seems to be not good. When I replaced to good cable, it has never happened. Thank you for your response. If same situation happened, I will try CyI3PI2cWaitForAck again.